1. Field of the Invention
The present invention relates to an electron gun of a cathode ray tube (hereinafter abbreviated as CRT) and a manufacturing method therefor.
2. Description of the Prior Art
The cathode cutoff voltage E.sub.KCO, which is one of the characteristics of an electron gun cathode structure, is the most important one among various characteristics of a CRT. It is very important to suppress a variation of the cathode cutoff voltage E.sub.KCO to obtain good characteristics of a CRT. For example, the cathode cutoff voltage E.sub.KCO depends on a distance d.sub.01 between a cathode and a first grid, a distance d.sub.12 between the first grid and a second grid, thicknesses t.sub.G1 and t.sub.G2 of the first and second grids, a diameter .PHI..sub.1 of an emission aperture of the first grid, a diameter .PHI..sub.2 of an emission aperture of the second gird, and a positional relationship between these emission apertures such that EQU E.sub.KCO =K.PHI..sub.1.sup.a .PHI..sub.2.sup.b /(d.sub.01.sup.c d.sub.12.sup.d t.sub.G1.sup.e t.sub.G2.sup.f)
where k, a, b, c, d, e and f are constants.
An electron gun of a CRT includes a cathode for emitting electrons and a plurality of electrodes for forming an electron beam from the emitted electrons and focusing the electron beam onto a phosphor screen while accelerating it to a high speed. For example, the present inventors' Japanese Patent Application No. Hei. 4-155765 describes an example of an electron gun.
The present inventors have proposed an electron gun in which variations of the distance d.sub.01 between a cathode and a first grid and the distances between the first grid and a second grid are reduced to thereby suppress a variation of the cathode cutoff voltage E.sub.KCO (Japanese Patent Application Unexamined Publication No. Hei. 5-36360). In this electron gun, a plurality of grids other than the first grid are supported by a pair of glass support bars so as to be arranged in order at predetermined intervals. Thus the electron gun is characterized in that the first grid is fixed to an insulating support member, and is attached to the second grid through a spacer which defines the distance between the first and second grids. For example, the first grid is fixed to the support member by silver brazing.
FIG. 1 is a schematic sectional view showing an arrangement of a first grid G.sub.1, an insulating support member 10, a spacer 12 and a cathode of the electron gun disclosed in the above-mentioned publication Hei. 5-36360. The cathode consists of a generally cylindrical sleeve 20, a cap 22 covering a tip portion of the sleeve 20, and an oxide material 24 as an emission source provided on top of the cap 22. A heater 26 is disposed in the generally cylindrical sleeve, whose bottom portion is a little larger in diameter than its top portion. A sleeve support member 28, which is cylindrical, is attached to the face of the support member 10 that is opposite to the face to which the spacer 12 is attached. The sleeve 20 is fixed to the sleeve support member 28. A second grid G.sub.2 is attached to the top face of the spacer 12. A though-hole 10a is formed in the support member 10 so as to correspond to an emission aperture G.sub.e of the first grid G.sub.1.
The distance between the top face of the spacer 12 and the top face of the first grid G.sub.1 corresponds to the distance d.sub.12. The distance between the bottom face of the first grid G.sub.1 and the cathode (specifically, the oxide material 24) corresponds to the distance d.sub.01.
In an ordinary CRT, it takes about 30 minutes for an operation of the CRT to reach the steady state from the start, i.e., turning on of the heater 26 of the cathode structure. During this period, heat radiation and conduction from the heater 26 makes the sleeve 20, support member 10 and first grid G.sub.x thermally expand to cause deformation of the first grid G.sub.1. This deformation usually originates from the fact that a thermal expansion .DELTA.L.sub.s /L.sub.L.sub.s of a material constituting the support member 10 is smaller than a thermal expansion .DELTA.L.sub.G /L.sub.G of a material constituting the first grid G.sub.1 due to the heat coming from the cathode.
As is well known in the art, the term ".DELTA.L.sub.S /L.sub.S " simply means the length "L" for a certain given dimension of the support member in a given direction, and for that length "L" in a certain direction, the change of length .DELTA.L which occurs at a certain temperature compared to room temperature. The same is also true for the first grid thermal expansion .DELTA.L.sub.G /L.sub.G.
The thermal expansion .DELTA.L.sub.S /L.sub.S can be expressed as a.sub.S .DELTA.t.sub.S where a.sub.S is a linear expansion coefficient of the material constituting the support member 10 and .DELTA.t.sub.S is a difference between the temperature of the support member 10 being subjected to the heat from the cathode and the room temperature. Similarly, the thermal expansion .DELTA.L.sub.G /L.sub.G can be expressed by a.sub.G .DELTA.t.sub.G where a.sub.G is a linear expansion coefficient of the material constituting the first grid G.sub.1 and .DELTA.t.sub.G is a difference between the first grid G.sub.1 being subjected to the heat from the cathode and the room temperature. The above notation is also employed in the following description. In some cases, the thermal expansion .DELTA.L.sub.S /L.sub.S of the material constituting the support member 10 and the thermal expansion .DELTA.L.sub.G /L.sub.G of the material constituting the first grid G.sub.1 are simply expressed as a thermal expansion of the support member 10 and a thermal expansion of the first grid G.sub.1 , respectively.
When the support member 10 and the first grid G.sub.1 thermally expand due to the heat radiation and conduction from the cathode, the thermal expansion of the first grid G.sub.1 is larger than that of the support member 10. Therefore, as schematically shown in FIG. 2, the first grid G.sub.1 is deformed so as to become convex upward, so that the distance d.sub.01 between the cathode and the first grid G.sub.1 and the distance d.sub.12 between the first grid G.sub.1 and the second grid G.sub.2 are varied. Further, a relative positional relationship between the cathode and the emission apertures of the first and second grids is also varied. As a result, there occurs a change of the cathode cutoff voltage E.sub.KCO, a large movement of a beam spot and a temporal variation of the luminance of a CRT screen.
To avoid positional deviations of the emission apertures of the first grid G.sub.1 and the second grid G.sub.1 , an electron gun is usually assembled by using the emission apertures themselves as a guide or by using proper guide holes. However, during heat treatments after the electron gun assembling, such as those in a CRT baking step and a gun heating step, the first grid G.sub.1 is deformed due to the difference in thermal expansion between the support member 10 and the first grid G.sub.1. As a result, the distance d.sub.01 between the cathode and the first grid G.sub.1 and the distance d.sub.12 between the first grid G.sub.1 and the second grid G.sub.2 are varied. Further, a relative positional relationship between the cathode and the emission apertures of the first and second grids is also varied. The heat treatments also vary the cathode cutoff voltage E.sub.KCO in the above manner.
In order to keep the distance d.sub.12 constant, another conventional technique employs a structure in which a spacer 111 made of an insulating material is inserted between a first grid 112 and a second grid 113 (see FIG. 3A). An electron gun cathode structure shown in FIG. 3A is produced such that both surfaces of the spacer 111 are metalized in advance, and the opposing surfaces of the first grid 112 and the spacer 111 and the opposing surfaces of the spacer 111 and the second grid 113 are brazed to each other in a state such that a shaft 14a of a brazing jig 14 is inserted into the first grid 112, spacer 111 and second grid 113.
However, in the electron gun cathode structure shown in FIG. 3A, the distance d.sub.12 is varied by variations of the thickness of the brazing material and the thicknesses of the metal layers formed by the metalizing treatment, variations in the flatness of the first grid 112 and the second grid 113, and other factors.
To solve this problem, an electron gun cathode structure shown in FIG. 3B has been proposed in which a metal spacer 15 and a first grid 16 are fixed to the same flat surface 17a of a support member 17 made of an insulating material, and a second grid (not shown) is fixed to a surface 15a of the spacer 15 opposite to the surface that is fixed to the support member 17. In this electron gun cathode structure, a step between the surface 16a of the first grid 16 and the surface 15a of the spacer 15 opposite to the surface that is fixed to the support member 17 corresponds to the distance d.sub.12.
The electron gun cathode structure of FIG. 3B is manufactured as follows. First, the flat surface 17a of the support member 17 is metalized. After shafts 18a of a brazing jig 18 are inserted into the support member 17, the first grid 16 and the spacer 15 are placed on the flat surface 17a. Then, the opposing surfaces of the first grid 16 and the support member 17 and the opposing surfaces of the spacer 15 and the support member 17 are brazed to each other while the first grid 16 and the spacer 15 are pressed by pressing portions 18b.
The distance d.sub.12 appears to be set more correctly in the electron gun cathode structure of FIG. 3B than in that of FIG. 3A. However, in the manufacturing method of the electron gun cathode structure shown in FIG. 3B, no proper measures are taken to suppress variations in the flatness of the support member 17, first grid 16 and spacer 15, variations of the thickness of the brazing material and the thickness of the metal layer formed by the metalizing treatment, variations of the thicknesses of the spacer 15 and the first grid 16, and other factors. Therefore, the distance d.sub.12 is not necessarily set correctly, failing to suppress the variation of the cathode cutoff voltage E.sub.KCO to a small value.
Considering the above, the present inventors have proposed a novel manufacturing method of an electron gun cathode structure (Japanese Patent Application Unexamined Publication No. Hei. 5-166457), which will be described later in detail. This manufacturing method allows the distance d.sub.12 to be set correctly to a certain extent.
However, to improve the characteristics of an electron gun cathode structure, it is now desired to set the distance d.sub.12 more correctly without a variation.
Conventionally, the distance d.sub.01 between the cathode and the first grid G.sub.1 is adjusted in the following manner using an air-micro device as a non-contact type distance measuring instrument. As shown in FIG. 4A, a nozzle portion 54 is inserted through the emission aperture G.sub.e of the first grid G.sub.1 with a reference surface 52 of an air-micro device 50 contacted with the top surface of the first grid G.sub.1. A pressurized gas such as a nitrogen gas or an air is jetted from the nozzle portion 54 of the air-micro device 50 to the oxide material 24 of the cathode. Since there exists a certain relationship between back pressure of the pressurized gas and the distance between the nozzle portion 54 and the oxide material 24. The distance between the reference surface 52 and the cathode can be obtained by measuring the back pressure with a gauge 56.
To improve the characteristics of CRTs, the diameters of the emission apertures of the respective grids now tend to decrease. Therefore, to maintain the cathode cutoff voltage E.sub.KCO at the same value, it is necessary to reduce the thickness of the grids.
However, for example, where the thickness of the first grid G.sub.1 is reduced, the first grid G.sub.1 is likely deformed when the reference surface 52 of the air-micro device 50 is contacted with the top surface of the first grid G.sub.1 (see FIG. 4B). This will cause a problem that the distance d.sub.01 between the first grid G.sub.1 and the cathode cannot be measured precisely. As a result, there occurs a variation of the distance d.sub.01, which means a variation of E.sub.KCO, after assembling of an electron gun cathode structure.